Routing & IP Routing Protocols

Routing: Problem Definition You are a router in a packet switched network and you receive a packet destined to some remote node E.g., router A below receives a packet destined to node F Question: How does A know where to send this packet? Does A send it to B? C? or D? Answer: Recall from our earlier discussion in IP that router A consults a forwarding table to make this decision Routing Problem: How does router A build this forwarding table? Built by a routing algorithm (protocol): The job of the routing algorithm is to determine the next hop router for ALL destinations in the network A E D C B F 2 1 3 5 Dest. Next Hop Cost B B 2 D D 1 C D 3 E D 2 F D 4 Forwarding Table in A

End-to-End Path Determination: Routing principles Routing protocol A E D C B F 2 1 3 5 Goal: determine “good” path (sequence of routers) thru network from source to dest. Graph abstraction for routing algorithms: graph nodes are routers graph edges are physical links link cost: delay, $ cost, or congestion level (amount of traffic carried on the link) “good” path: typically means minimum cost path

Forwarding vs Routing Distinction between “Forwarding” and “Routing” Forwarding consists of taking a packet, looking at its destination address, consulting the forwarding table, and sending the packet in a direction determined by the table Very easy once the forwarding table has been built Routing is the process by which the forwarding table is built Need a routing protocol to dynamically build and maintain the table Dest. Next Hop Cost B B 2 D D 1 C D 3 E D 2 F D 4 Forwarding table in A A E D C B F 2 1 3 5

Routing Algorithm classification Global or decentralized information? Global: all routers have complete topology, link cost info “link state” algorithms Decentralized: router knows physically-connected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors “distance vector” algorithms

Link-State Algorithms: General Idea Have each router build the complete topology of the network Once the complete topology is built, have each router run an algorithm to compute the shortest path from itself to all other routers (nodes) in the network 2 Issues How does a router build the complete topology of the network? How does a router compute the shortest path to all other nodes in the network using this topology information? Dijkstra’s Shortest Path Algorithm A E D C B F 2 1 3 5

Building the Network Topology At the heart of a link state algorithm is the discovery of a node’s links’ states Each node is assumed to capable of finding out the state of the link to its neighbors (up or down) and the cost of each link Each node creates an update packet, also called a link-state packet (LSP) and periodically sends this information to all of its neighbors Node’s neighbors send the packet to their neighbors and so on until the LSP is received by all nodes in the network LSP Contents ID of the node -- A List of neighbors and the cost to each neighbor – (B, 2), (C, 5), (D, 1) A sequence number A time-to-live (TTL) A E D C B F 2 1 3 5

Reliable Flooding Reliable flooding is the process of making sure that all nodes get a copy of LSP from all other nodes in the network When a node X receives a copy of an LSP that originated at some other node Y, it checks to see if it has already stored a copy of an LSP from Y. If not, it stores the LSP. If it already has a copy, it compares the SeqNos; if the new LSP has a larger SeqNo, it is assumed to be more recent, and the last LSP is stored replacing the old one. Otherwise the new LSP is discarded A E D C B F 2 1 3 5 The new LSP is then forwarded on all neighbors of X except the neighbor from which the LSP was just received

Computing the Shortest Path: Dijkstra’s Algorithm Notation: Cost(i,j): link cost from node i to j. Cost(A, B) = 2 Cost(A, C) = 5 Cost(A, F) = infinity Distance(v): current value of cost of path from source to destination V Distance(D) = 1 Pred(v): predecessor node of v along path from source to v Pred(D) = A N: set of nodes whose least cost path definitively known A E D C B F 2 1 3 5

Dijsktra’s Algorithm – where A is the source node 1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then Distance (v) = cost(A,v) 6 else Distance (v) = infinity 7 8 Loop 9 find w not in N such that Distance(w) is a minimum 10 add w to N 11 update Distance(v) for all v adjacent to w and not in N: 12 Distance(v) = min( Distance(v), Distance(w) + cost(w,v) ) 13 /* new distance to v is either old distance to v or known 14 shortest path distance to w plus cost from w to v */ 15 until all nodes in N

Dijkstra’s algorithm: example Step 1 2 3 4 5 N A AD ADE ADEB ADEBC ADEBCF D(B),p(B) 2,A D(C),p(C) 5,A 4,D 3,E D(D),p(D) 1,A D(E),p(E) infinity 2,D D(F),p(F) infinity 4,E 5 3 B C 5 2 A 2 1 F 3 1 2 D E 1 Algorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n*(n+1)/2 comparisons: O(n2) – O(nlogn) algorithm possible

Distance Vector Algos: General Idea We do NOT need to know the complete topology of the network to build the forwarding table! If a node X simply tells its neighbor Y the cost of reaching another node Z via itself (X), then neighbor Y can compute the cost of reaching Z via X by simply adding the cost of reaching X from Y and the cost of reaching Z from X, which was advertised by X Example If D tells A that it can reach E with a cost of 1, then A knows it can reach E via D with a cost of 1+1 = 2 If D tells A that it can reach F with a cost of 3, then A knows it can reach F via D with a cost of 1+3 = 4 A E D C B F 2 1 3 5

Distance Vector Routing Algorithm Each node tells its neighbors the cost of reaching every other node via itself A node computes its cost of reaching a destination via each of its neighbors and picks the best one distributed: each node communicates only with directly-attached neighbors iterative: continues until no nodes exchange new information self-terminating: no “signal” to stop D (Y,Z) X distance from X to Y via Z as next hop c(X,Z) + D (Z) Y = D (Y) min {D (Y,w)} w D (Z) Y C(X,Y) X Y Z C(X,T) T D (Z) T

Distance Table Structure Distance Table Data Structure each node has its own distance table row for each possible destination column for each directly-attached neighbor to node example: in node X, for dest. Y via neighbor Z: D () A B C D 1 7 6 4 14 8 9 11 5 2 E cost to destination via destination A E D C B 7 8 1 2

Distance Table gives Forwarding Table 1 7 6 4 14 8 9 11 5 2 E cost to destination via destination Outgoing link to use, cost A B C D A,1 D,5 D,4 destination Distance Table Forwarding (Routing) Table

Distance Vector Routing: overview Each node: wait for (msg from neighbor) recompute distance table if least cost path to any dest has changed, notify neighbors Initialize the Distance Table Structure

Initialization At all nodes, X: 1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infinity /* the * operator means "for all rows" */ 4 D (v,v) = c(X,v) 5 for all destinations, y 6 send D (y) to each neighbor /* Cost of reaching Y via X */ X X X

Distance Vector Algorithm: Example We need to adjust values if necessary X Z 1 2 7 Y This column shows the Init. results Init Info sent

Distance Vector Algorithm (cont.): 8 loop 9 wait (until I receive update from neighbor V) 10 if (update received from neighbor V wrt destination Y) 11 /* shortest path from V to some Y has changed */ 12 /* V has sent a new value for its DV(Y) */ 13 /* call this received new value is "newval" */ 14 for the single destination Y: D (Y,V) = c(X,V) + newval 15 16 if we have a new D (Y) for any destination Y 17 send new value of D (Y) to all neighbors 18 19 forever X X X

Distance Vector Algorithm: Example info sent Adjusted new value Initial values X Z 1 2 7 Y D (Z,Y) X c(X,Y) + D (Z) = 2+1 = 3 Y D (Y,Z) X c(X,Z) + D (Y) = 7+1 = 8 Z

Distance Vector Algorithm: Example Adjust values if necessary This column Shows the Init. results Init Info sent Adjusted new values New Info sent X Z 1 2 7 Y

Hierarchical Routing 1. scale: with 200 million destinations: Our routing study thus far assumed all routers to be “identical” and the network to be “flat” 2 Problems exist with such a model: 1. scale: with 200 million destinations: can’t store all destinations in routing tables! routing table exchange would swamp links! 2. administrative autonomy Can’t assume that a network with the scale of Internet will be administered by a single authority Solution? Internet is NOT flat, but is a network of networks each network admin controls routing in its own network – next http://www.ilkertemir.com/backbone-map/ Turkey’s past and current Internet backbone map

Hierarchical Routing gateway routers C.b B.a A.a b c A.c a a C b a B d special routers in AS run intra-AS routing protocol with all other routers in AS also responsible for routing to destinations outside AS run inter-AS routing protocol with other gateway routers aggregate routers into regions, “autonomous systems” (AS) routers in same AS run same routing protocol “intra-AS” routing protocol routers in different AS can run different intra-AS routing protocol

Intra-AS Routing Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols: OSPF: Open Shortest Path First RIP: Routing Information Protocol IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

OSPF (Open Shortest Path First) “open”: publicly available Uses Link State algorithm LS advertisement dissemination to entire AS via flooding Topology map at each node Route computation using Dijkstra’s algorithm Carried in OSPF messages directly over IP OSPF has its own network layer protocol number like IP!

RIP (Routing Information Protocol) Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops) 16 is the infinity to eliminate routing loops Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement) Each advertisement: list of up to 25 destination nets within AS If more destinations, send multiple advertisements

RIP: Example z w x y A D B C y B 2 z B 7 x -- 1 Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B 7 x -- 1 …. …. .... Routing table in D

RIP: Example w x y z A C D B y B 2 z B A 7 5 x -- 1 Advertisement Dest Next hops w - - x - - z C 4 …. … ... Advertisement from A to D w x y z A C D B Destination Network Next Router Num. of hops to dest. w A 2 y B 2 z B A 7 5 x -- 1 …. …. .... Routing table in D

RIP Table processing RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated routed routed Transprt (UDP) Transprt (UDP) network forwarding (IP) table network (IP) forwarding table link link physical physical

Inter-AS routing in the Internet: BGP Each BGP router communicates only with its neighbors R1 with R2, R3 with R4 Global info about routes to destination networks propagates in an AS-by-AS manner via the exchange of BGP messages

Why different Intra- and Inter-AS routing ? Policy: Intra-AS: single admin, so no policy decisions needed Inter-AS: admin wants control over how its traffic routed, who routes through its net. Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance